Batteries typically have a relatively large output voltage range, with the output voltage falling during the life of the battery. FIG. 1A illustrates the output voltage variation of a single typical alkaline battery cell over its life.
Unlike the batteries, ICs typically do not have operating voltage ranges that match the output voltage range of batteries. A common operating voltage range of an IC may be from about 2.4V to about 3.6V. Such an IC may be powered from a pair of alkaline batteries having a combined output voltage in the range approximately of 1.8V to 3.6V. Thus, there is a mismatch between the voltage ranges of the IC and the batteries.
One conventional technique to overcome this voltage range mismatch is to include a Direct Current (DC) to DC boost converter and a voltage detector in the IC. If the battery voltage is greater than the minimum voltage specified for the IC, then the boost converter is disabled and the IC is powered directly from the batteries. If the battery voltage falls below the specified minimum voltage, then the boost converter is enabled and the IC is powered from the boost converter instead. In many cases, the boost converter automatically supplies the battery voltage directly to the IC when the battery voltage is greater than the programmed output voltage. The boost converter and the IC powered by the boost converter may or may not be separate devices.
FIG. 1B illustrates an existing boost converter. Referring to FIG. 1B, the reference voltage 110 is substantially fixed to control voltage regulation. Specifically, when the output voltage of the boost converter 100 falls below the specified output voltage, a switching circuit 120 is activated to cause the output voltage 130 to rise above the specified output voltage.
However, the given minimum operating voltage, also referred to as the guaranteed minimum operating voltage Vcc(min), may not be the actual minimum voltage at which the IC can operate correctly under certain circumstances. Many ICs typically operate correctly somewhat below Vcc(min) in practice. In order to guarantee operation at the specified Vcc(min), IC manufacturers typically test the operation of the IC both slightly above the maximum rated temperature, and slightly below the minimum rated temperature at a voltage somewhat below Vcc(min). The voltage at which the IC ceases to operate correctly is generally below Vcc(min). In many cases, such a voltage may be well below Vcc(min) if the device is operated at room temperature.
In a sample case, an IC may be rated with Vcc(min) being approximately 2.7V and Vcc(max) being approximately 3.6V, with an operating temperature range of 0 to 70 degrees Celsius. The IC manufacturer may test every IC at about 2.6V at both −10 and +80 degrees Celsius. In this example, the IC may work correctly in all respects down to about 2.5V, and with degraded performance down to about 2.3V, provided that the temperature of operation is constrained to a smaller range than that specified (e.g., 0 to 70 degrees Celsius), such as 10 to 40 degrees Celsius.